Allow user to add postmeta

I would like to know if its possible for me to let my user to add postmeta using submit form and if there already plugin for this please provide me a link to it.

Here is what I mean by what I said above. In my theme I will add this:

<?php
		$dload = get_post_custom_values('download');
		if (empty ($dload)){echo "";} else {
		echo "<strong>Download Link:</strong><br />";
		foreach ( $dload as $key => $value ) {
				if (stripos($value,'/uploading.com/') !== false) {
					echo "<a href=" . $value . ">Uploading</a><br />";
				} else {
					echo "<a href=" . $value . ">Other</a><br />";
				}
			}
		}
	?>
<!-- and so on this is sample only -->

So I need now is to let logged in user to be able to post (like posting a comment) to add URL and submit. I can do this in a stone-age way but I need an advise for efficient to do this.

The tricky part is how can I include username along with the link he posted eg:

CrumX [ Uploading ] // (imagine that [ ] is a button)
AnotherX [ Rapidshare ]
You [ Megaupload ]

The point here is I dont want to let signed in user back end editing capabilities.